Ground fault protection of gas lamp power supplies

Abstract

A power supply for powering a neon sign assembly includes a ground fault detection circuit having a current sensing circuit that produces a voltage that is proportional to the current that flows from a center tap of an output transformer, a phase-sensitive rectifier that receives output from the current sensing circuit and a reference voltage from the transformer and produces an output voltage that depends on the magnitude of the output from the current sensing circuit and the phase angle between the output from the current sensing circuit and the reference voltage, and an amplitude comparator that produces an output to a latch circuit only when an output voltage from the phase-sensitive rectifier is equal to or greater than a pre-set ground fault current limit.

What is claimed is: 
     
       1. A ground fault protection apparatus for a power supply having a transformer, the apparatus comprising: a current sensing element for producing a voltage proportional to a current from a center tap of the transformer;   a phase-sensitive rectifier for producing a voltage V o  based on a reference voltage from the transformer and the voltage from the current sensing element;   an amplitude comparator for producing an output when the voltage from the phase-sensitive rectifier is equal to or greater than a pre-set value corresponding to a ground fault current limit, the amplitude comparator including a first comparator for responding to a positive voltage from the phase-sensitive rectifier and a second comparator for responding to a negative voltage from the phase-sensitive rectifier; and   a latch circuit for inhibiting operation of the power supply when the output from the amplitude comparator is supplied thereto,   wherein the voltage produced by the phase-sensitive rectifier corresponds to   V.sub.o =V.sub.s cosφ,     where V s  represents the output from the current sensing element and φ represents a phase angle between V s  and the reference voltage from the transformer.     
     
     
       2. An apparatus according to claim 1, wherein the current sensing element comprises a current transformer and an associated resistor. 
     
     
       3. An apparatus according to claim 1, wherein the current sensing element comprises one of a resistor or a current shunt. 
     
     
       4. An apparatus according to claim 1, wherein the current sensing element comprises a Hall effect device. 
     
     
       5. An apparatus according to claim 1, wherein the current sensing element comprises a capacitor. 
     
     
       6. An apparatus according to claim 1, wherein the current sensing element comprises an inductor. 
     
     
       7. An apparatus according to claim 1, wherein the phase-sensitive rectifier is configured as a half-wave rectifier and includes an analog bi-directional gate circuit. 
     
     
       8. An apparatus according to claim 1, wherein the phase-sensitive rectifier is configured as a full-wave rectifier and includes an analog bi-directional gate circuit. 
     
     
       9. An apparatus according to claim 1, wherein the phase-sensitive rectifier is configured as a half-wave rectifier and includes a transistor operating as a gate. 
     
     
       10. An apparatus according to claim 1, wherein the phase-sensitive rectifier is configured as a full-wave rectifier and includes a transistor operating as a gate. 
     
     
       11. An apparatus according to claim 1, wherein the amplitude comparator outputs fault condition data, and   the latch circuit includes a memory for storing the fault condition data.   
     
     
       12. An apparatus according to claim 1, wherein the output from the amplitude comparator causes the latch circuit to temporarily inhibit the operation of the power supply to enable the power supply to automatically restart. 
     
     
       13. An apparatus according to claim 12, wherein the latch circuit includes limiting means for preventing the power supply from automatically restarting when the power supply has been automatically restarted a pre-defined number of times. 
     
     
       14. An apparatus according to claim 1, wherein the phase-sensitive rectifier produces an output indicating a capacitive imbalance based on a magnitude and polarity of the voltage from the current sensing element, and   the output from the phase-sensitive rectifier causes the amplitude comparator to supply an output signal to the latch circuit.   
     
     
       15. A method of protecting a power supply including a transformer from ground faults, the method comprising the steps of: determining a current from a center tap of the transformer;   converting the current from the center tap of the transformer to a voltage;   obtaining a reference voltage from the transformer;   producing a rectified voltage based on the converted voltage and the obtained reference voltage;   supplying the rectified voltage to a first comparator when the voltage is a positive voltage;   supplying the rectified voltage to a second comparator when the voltage is a negative voltage; and   feeding a signal output from the first comparator or the second comparator to a latch circuit to inhibit operation of the power supply when the rectified voltage is greater than a predetermined limit that corresponds to a ground fault current limit,   wherein the rectified voltage discriminates between a true ground current and a current due to capacitive imbalance based on a phase angle between the converted voltage and the reference voltage.